Image encoding/decoding method and apparatus for performing intra prediction, and method for transmitting bitstream

ABSTRACT

An image encoding/decoding method and apparatus are provided. An image decoding method according to the present disclosure comprises determining whether to apply intra prediction to a current chroma block based on information on prediction of the current chroma block, deriving an intra prediction mode of the current chroma block based on an intra prediction mode of a corresponding luma block corresponding to the current chroma block and intra chroma prediction mode information of the current chroma block, when intra prediction applies to the current chroma block, and generating a prediction block of the current chroma block, by performing intra prediction based on the intra prediction mode of the current chroma block. When the intra prediction mode of the corresponding luma block is not present, the intra prediction mode of the current chroma block is derived based on a default intra prediction mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(e), this application is a continuation ofInternational Application PCT/KR2020/003562, with an internationalfiling date of Mar. 13, 2020, which claims the benefit of U.S.Provisional Patent Application No. 62/818,730, filed on Mar. 14, 2019,the contents of which are hereby incorporated by reference herein intheir entirety.

TECHNICAL FIELD

The present disclosure relates to an image encoding/decoding method andapparatus, and, more particularly, to a method and apparatus forencoding/decoding an image using intra prediction, and a method oftransmitting a bitstream generated by the image encodingmethod/apparatus of the present disclosure.

BACKGROUND ART

Recently, demand for high-resolution and high-quality images such ashigh definition (HD) images and ultra high definition (UHD) images isincreasing in various fields. As resolution and quality of image dataare improved, the amount of transmitted information or bits relativelyincreases as compared to existing image data. An increase in the amountof transmitted information or bits causes an increase in transmissioncost and storage cost.

Accordingly, there is a need for high-efficient image compressiontechnology for effectively transmitting, storing and reproducinginformation on high-resolution and high-quality images.

DISCLOSURE Technical Problem

An object of the present disclosure is to provide an imageencoding/decoding method and apparatus with improved encoding/decodingefficiency.

Another object of the present disclosure is to provide a method andapparatus for encoding/decoding an image using intra prediction.

Another object of the present disclosure is to provide an imageencoding/decoding method and apparatus for performing intra predictionwith respect to a chroma block after an intra prediction mode of thechroma block is derived based on an intra prediction mode of acorresponding luma block or a default intra prediction mode.

Another object of the present disclosure is to provide a method oftransmitting a bitstream generated by an image encoding method orapparatus according to the present disclosure.

Another object of the present disclosure is to provide a recordingmedium storing a bitstream generated by an image encoding method orapparatus according to the present disclosure.

Another object of the present disclosure is to provide a recordingmedium storing a bitstream received, decoded and used to reconstruct animage by an image decoding apparatus according to the presentdisclosure.

The technical problems solved by the present disclosure are not limitedto the above technical problems and other technical problems which arenot described herein will become apparent to those skilled in the artfrom the following description.

Technical Solution

An image decoding method according to an aspect of the presentdisclosure may comprise determining whether to apply intra prediction toa current chroma block based on information on prediction of the currentchroma block, deriving an intra prediction mode of the current chromablock based on an intra prediction mode of a corresponding luma blockcorresponding to the current chroma block and intra chroma predictionmode information of the current chroma block, when intra predictionapplies to the current chroma block, and generating a prediction blockof the current chroma block, by performing intra prediction based on theintra prediction mode of the current chroma block. When the intraprediction mode of the corresponding luma block is not present, theintra prediction mode of the current chroma block may be derived basedon a default intra prediction mode.

In the image decoding method according to the present disclosure, thederiving the intra prediction mode of the current chroma block maycomprise determining a prediction method at a predetermined position ofthe corresponding luma block.

In the image decoding method according to the present disclosure, whenthe prediction method at the predetermined position of the correspondingluma block is intra prediction, the intra prediction mode of the currentchroma block may be derived based on an intra prediction mode at apredetermined position of the corresponding luma block, and, when theprediction method at the predetermined position of the correspondingluma block is not intra prediction, the intra prediction mode of thecurrent chroma block may be derived based on the default intraprediction mode.

In the image decoding method according to the present disclosure, whenthe prediction method at the predetermined position of the correspondingluma block is intra block copy (IBC) prediction, the intra predictionmode of the current chroma block may be derived based on the defaultintra prediction mode.

In the image decoding method according to the present disclosure, thepredetermined position may be a center position of the correspondingluma block.

In the image decoding method according to the present disclosure, thedefault intra prediction mode may be a planar mode or a DC mode.

In the image decoding method according to the present disclosure, a treestructure of the current chroma block may be a dual tree (DUAL_TREE)structure.

In the image decoding method according to the present disclosure, whenthe intra prediction mode information of the current chroma blockindicates a direct mode (DM) and the intra prediction mode of thecorresponding luma block is present, the intra prediction mode of thecurrent chroma block may be derived as the intra prediction mode of thecorresponding luma block, and, when the intra prediction modeinformation of the current chroma block indicates a DM and the intraprediction mode of the corresponding luma block is not present, theintra prediction mode of the current chroma block may be derived as thedefault intra prediction mode.

An image decoding apparatus according to another aspect of the presentdisclosure comprises a memory, and at least one processor. The at leastone processor may determine whether to apply intra prediction to acurrent chroma block based on information on prediction of the currentchroma block, derive an intra prediction mode of the current chromablock based on an intra prediction mode of a corresponding luma blockcorresponding to the current chroma block and intra chroma predictionmode information of the current chroma block, when intra predictionapplies to the current chroma block, and generate a prediction block ofthe current chroma block, by performing intra prediction based on theintra prediction mode of the current chroma block. When the intraprediction mode of the corresponding luma block is not present, theintra prediction mode of the current chroma block may be derived basedon a default intra prediction mode.

An image encoding method according to another aspect of the presentdisclosure comprises determining whether to apply intra prediction to acurrent chroma block, deriving an intra prediction mode of the currentchroma block based on an intra prediction mode of a corresponding lumablock corresponding to the current chroma block, when intra predictionapplies to the current chroma block, generating a prediction block ofthe current chroma block, by performing intra prediction based on theintra prediction mode of the current chroma block, and encoding theintra prediction mode of the current chroma block based on the intraprediction mode of the corresponding luma block. When the intraprediction mode of the corresponding luma block is not present, theintra prediction mode of the current chroma block may be derived basedon a default intra prediction mode.

In the image encoding method according to the present disclosure, thederiving the intra prediction mode of the current chroma block maycomprise determining a prediction method at a predetermined position ofthe corresponding luma block.

In the image encoding method according to the present disclosure, whenthe prediction method at the predetermined position of the correspondingluma block is intra prediction, the intra prediction mode of the currentchroma block may be derived based on an intra prediction mode at apredetermined position of the corresponding luma block, and, when theprediction method at the predetermined position of the correspondingluma block is not intra prediction, the intra prediction mode of thecurrent chroma block may be derived based on the default intraprediction mode.

In the image encoding method according to the present disclosure, thepredetermined position may be a center position of the correspondingluma block.

In the image encoding method according to the present disclosure, thedefault intra prediction mode may be a planar mode or a DC mode.

In addition, a transmission method according to another aspect of thepresent disclosure may transmit the bitstream generated by the imageencoding apparatus or the image encoding method of the presentdisclosure.

In addition, a computer-readable recording medium according to anotheraspect of the present disclosure may store the bitstream generated bythe image encoding apparatus or the image encoding method of the presentdisclosure.

The features briefly summarized above with respect to the presentdisclosure are merely exemplary aspects of the detailed descriptionbelow of the present disclosure, and do not limit the scope of thepresent disclosure.

Advantageous Effects

According to the present disclosure, an image encoding/decoding methodand apparatus with improved encoding/decoding efficiency may beprovided.

According to the present disclosure, a method and apparatus forencoding/decoding an image using intra prediction may be provided.

According to the present disclosure, an image encoding/decoding methodand apparatus for performing intra prediction with respect to a chromablock after an intra prediction mode of the chroma block is derivedbased on an intra prediction mode of a corresponding luma block or adefault intra prediction mode may be provided.

Also, according to the present disclosure, a method of transmitting abitstream generated by an image encoding method or apparatus accordingto the present disclosure may be provided.

Also, according to the present disclosure, a recording medium storing abitstream generated by an image encoding method or apparatus accordingto the present disclosure may be provided.

Also, according to the present disclosure, a recording medium storing abitstream received, decoded and used to reconstruct an image by an imagedecoding apparatus according to the present disclosure may be provided.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present disclosure are notlimited to what has been particularly described hereinabove and otheradvantages of the present disclosure will be more clearly understoodfrom the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing a video coding system, to whichan embodiment of the present disclosure is applicable.

FIG. 2 is a view schematically showing an image encoding apparatus, towhich an embodiment of the present disclosure is applicable.

FIG. 3 is a view schematically showing an image decoding apparatus, towhich an embodiment of the present disclosure is applicable.

FIG. 4 is a view showing a partitioning type of a block according to amulti-type tree structure.

FIG. 5 is a view showing a signaling mechanism of partition splittinginformation in a quadtree with nested multi-type tree structureaccording to the present disclosure.

FIG. 6 is a flowchart illustrating an intra prediction based video/imageencoding method.

FIG. 7 is a view showing the configuration of the intra prediction unit185 according to the present disclosure.

FIG. 8 is a flowchart illustrating an intra prediction based video/imagedecoding method.

FIG. 9 is a view showing the configuration of the intra prediction unit265 according to the present disclosure.

FIG. 10 is a flowchart illustrating an intra prediction mode signalingprocedure in an image encoding apparatus.

FIG. 11 is a flowchart illustrating an intra prediction modedetermination procedure in an image decoding apparatus.

FIG. 12 is a flowchart illustrating an intra prediction mode derivationprocedure in more detail.

FIG. 13 is a view showing an intra prediction direction according to anembodiment of the present disclosure.

FIG. 14 is a view showing an intra prediction direction according toanother embodiment of the present disclosure.

FIG. 15 is a view showing a predetermined position for deriving an intraprediction mode of a current chroma block in a DM mode.

FIG. 16 is a flowchart illustrating a conventional method of deriving anintra prediction mode of a chroma block based on a corresponding lumablock.

FIG. 17 is a flowchart illustrating an embodiment of the presentdisclosure of deriving an intra prediction mode of a chroma block basedon a corresponding luma block.

FIG. 18 is a flowchart illustrating another embodiment of the presentdisclosure of deriving an intra prediction mode of a chroma block basedon a corresponding luma block.

FIG. 19 is a flowchart illustrating another embodiment of the presentdisclosure of deriving an intra prediction mode of a chroma block basedon a corresponding luma block.

FIG. 20 is a flowchart illustrating another embodiment of the presentdisclosure of deriving an intra prediction mode of a current block basedon a corresponding block.

FIG. 21 is a flowchart illustrating another embodiment of the presentdisclosure of deriving an intra prediction mode of a chroma block basedon a corresponding luma block and encoding the chroma block.

FIG. 22 is a view showing a content streaming system, to which anembodiment of the present disclosure is applicable.

MODE FOR INVENTION

Hereinafter, the embodiments of the present disclosure will be describedin detail with reference to the accompanying drawings so as to be easilyimplemented by those skilled in the art. However, the present disclosuremay be implemented in various different forms, and is not limited to theembodiments described herein.

In describing the present disclosure, if it is determined that thedetailed description of a related known function or construction rendersthe scope of the present disclosure unnecessarily ambiguous, thedetailed description thereof will be omitted. In the drawings, parts notrelated to the description of the present disclosure are omitted, andsimilar reference numerals are attached to similar parts.

In the present disclosure, when a component is “connected”, “coupled” or“linked” to another component, it may include not only a directconnection relationship but also an indirect connection relationship inwhich an intervening component is present. In addition, when a component“includes” or “has” other components, it means that other components maybe further included, rather than excluding other components unlessotherwise stated.

In the present disclosure, the terms first, second, etc. may be usedonly for the purpose of distinguishing one component from othercomponents, and do not limit the order or importance of the componentsunless otherwise stated. Accordingly, within the scope of the presentdisclosure, a first component in one embodiment may be referred to as asecond component in another embodiment, and similarly, a secondcomponent in one embodiment may be referred to as a first component inanother embodiment.

In the present disclosure, components that are distinguished from eachother are intended to clearly describe each feature, and do not meanthat the components are necessarily separated. That is, a plurality ofcomponents may be integrated and implemented in one hardware or softwareunit, or one component may be distributed and implemented in a pluralityof hardware or software units. Therefore, even if not stated otherwise,such embodiments in which the components are integrated or the componentis distributed are also included in the scope of the present disclosure.

In the present disclosure, the components described in variousembodiments do not necessarily mean essential components, and somecomponents may be optional components. Accordingly, an embodimentconsisting of a subset of components described in an embodiment is alsoincluded in the scope of the present disclosure. In addition,embodiments including other components in addition to componentsdescribed in the various embodiments are included in the scope of thepresent disclosure.

The present disclosure relates to encoding and decoding of an image, andterms used in the present disclosure may have a general meaning commonlyused in the technical field, to which the present disclosure belongs,unless newly defined in the present disclosure.

In the present disclosure, a “picture” generally refers to a unitrepresenting one image in a specific time period, and a slice/tile is acoding unit constituting a part of a picture, and one picture may becomposed of one or more slices/tiles. In addition, a slice/tile mayinclude one or more coding tree units (CTUs).

In the present disclosure, a “pixel” or a “pel” may mean a smallest unitconstituting one picture (or image). In addition, “sample” may be usedas a term corresponding to a pixel. A sample may generally represent apixel or a value of a pixel, and may represent only a pixel/pixel valueof a luma component or only a pixel/pixel value of a chroma component.

In the present disclosure, a “unit” may represent a basic unit of imageprocessing. The unit may include at least one of a specific region ofthe picture and information related to the region. The unit may be usedinterchangeably with terms such as “sample array”, “block” or “area” insome cases. In a general case, an M×N block may include samples (orsample arrays) or a set (or array) of transform coefficients of Mcolumns and N rows.

In the present disclosure, “current block” may mean one of “currentcoding block”, “current coding unit”, “coding target block”, “decodingtarget block” or “processing target block”. When prediction isperformed, “current block” may mean “current prediction block” or“prediction target block”. When transform (inversetransform)/quantization (dequantization) is performed, “current block”may mean “current transform block” or “transform target block”. Whenfiltering is performed, “current block” may mean “filtering targetblock”.

In addition, in the present disclosure, a “current block” may mean “aluma block of a current block” unless explicitly stated as a chromablock. The “chroma block of the current block” may be expressed byincluding an explicit description of a chroma block, such as “chromablock” or “current chroma block”.

In the present disclosure, the term “/” and “,” should be interpreted toindicate “and/or.” For instance, the expression “A/B” and “A, B” maymean “A and/or B.” Further, “A/B/C” and “A/B/C” may mean “at least oneof A, B, and/or C.”

In the present disclosure, the term “or” should be interpreted toindicate “and/or.” For instance, the expression “A or B” may comprise 1)only “A”, 2) only “B”, and/or 3) both “A and B”. In other words, in thepresent disclosure, the term “or” should be interpreted to indicate“additionally or alternatively.”

Overview of Video Coding System

FIG. 1 is a view showing a video coding system according to the presentdisclosure.

The video coding system according to an embodiment may include aencoding apparatus 10 and a decoding apparatus 20. The encodingapparatus 10 may deliver encoded video and/or image information or datato the decoding apparatus 20 in the form of a file or streaming via adigital storage medium or network.

The encoding apparatus 10 according to an embodiment may include a videosource generator 11, an encoding unit 12 and a transmitter 13. Thedecoding apparatus 20 according to an embodiment may include a receiver21, a decoding unit 22 and a renderer 23. The encoding unit 12 may becalled a video/image encoding unit, and the decoding unit 22 may becalled a video/image decoding unit. The transmitter 13 may be includedin the encoding unit 12. The receiver 21 may be included in the decodingunit 22. The renderer 23 may include a display and the display may beconfigured as a separate device or an external component.

The video source generator 11 may acquire a video/image through aprocess of capturing, synthesizing or generating the video/image. Thevideo source generator 11 may include a video/image capture deviceand/or a video/image generating device. The video/image capture devicemay include, for example, one or more cameras, video/image archivesincluding previously captured video/images, and the like. Thevideo/image generating device may include, for example, computers,tablets and smartphones, and may (electronically) generate video/images.For example, a virtual video/image may be generated through a computeror the like. In this case, the video/image capturing process may bereplaced by a process of generating related data.

The encoding unit 12 may encode an input video/image. The encoding unit12 may perform a series of procedures such as prediction, transform, andquantization for compression and coding efficiency. The encoding unit 12may output encoded data (encoded video/image information) in the form ofa bitstream.

The transmitter 13 may transmit the encoded video/image information ordata output in the form of a bitstream to the receiver 21 of thedecoding apparatus 20 through a digital storage medium or a network inthe form of a file or streaming. The digital storage medium may includevarious storage mediums such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, andthe like. The transmitter 13 may include an element for generating amedia file through a predetermined file format and may include anelement for transmission through a broadcast/communication network. Thereceiver 21 may extract/receive the bitstream from the storage medium ornetwork and transmit the bitstream to the decoding unit 22.

The decoding unit 22 may decode the video/image by performing a seriesof procedures such as dequantization, inverse transform, and predictioncorresponding to the operation of the encoding unit 12.

The renderer 23 may render the decoded video/image. The renderedvideo/image may be displayed through the display.

Overview of Image Encoding Apparatus

FIG. 2 is a view schematically showing an image encoding apparatus, towhich an embodiment of the present disclosure is applicable.

As shown in FIG. 2, the image encoding apparatus 100 may include animage partitioner 110, a subtractor 115, a transformer 120, a quantizer130, a dequantizer 140, an inverse transformer 150, an adder 155, afilter 160, a memory 170, an inter prediction unit 180, an intraprediction unit 185 and an entropy encoder 190. The inter predictionunit 180 and the intra prediction unit 185 may be collectively referredto as a “prediction unit”. The transformer 120, the quantizer 130, thedequantizer 140 and the inverse transformer 150 may be included in aresidual processor. The residual processor may further include thesubtractor 115.

All or at least some of the plurality of components configuring theimage encoding apparatus 100 may be configured by one hardware component(e.g., an encoder or a processor) in some embodiments. In addition, thememory 170 may include a decoded picture buffer (DPB) and may beconfigured by a digital storage medium.

The image partitioner 110 may partition an input image (or a picture ora frame) input to the image encoding apparatus 100 into one or moreprocessing units. For example, the processing unit may be called acoding unit (CU). The coding unit may be acquired by recursivelypartitioning a coding tree unit (CTU) or a largest coding unit (LCU)according to a quad-tree binary-tree ternary-tree (QT/BT/TT) structure.For example, one coding unit may be partitioned into a plurality ofcoding units of a deeper depth based on a quad tree structure, a binarytree structure, and/or a ternary structure. For partitioning of thecoding unit, a quad tree structure may be applied first and the binarytree structure and/or ternary structure may be applied later. The codingprocedure according to the present disclosure may be performed based onthe final coding unit that is no longer partitioned. The largest codingunit may be used as the final coding unit or the coding unit of deeperdepth acquired by partitioning the largest coding unit may be used asthe final coding unit. Here, the coding procedure may include aprocedure of prediction, transform, and reconstruction, which will bedescribed later. As another example, the processing unit of the codingprocedure may be a prediction unit (PU) or a transform unit (TU). Theprediction unit and the transform unit may be split or partitioned fromthe final coding unit. The prediction unit may be a unit of sampleprediction, and the transform unit may be a unit for deriving atransform coefficient and/or a unit for deriving a residual signal fromthe transform coefficient.

The prediction unit (the inter prediction unit 180 or the intraprediction unit 185) may perform prediction on a block to be processed(current block) and generate a predicted block including predictionsamples for the current block. The prediction unit may determine whetherintra prediction or inter prediction is applied on a current block or CUbasis. The prediction unit may generate various information related toprediction of the current block and transmit the generated informationto the entropy encoder 190. The information on the prediction may beencoded in the entropy encoder 190 and output in the form of abitstream.

The intra prediction unit 185 may predict the current block by referringto the samples in the current picture. The referred samples may belocated in the neighborhood of the current block or may be located apartaccording to the intra prediction mode and/or the intra predictiontechnique. The intra prediction modes may include a plurality ofnon-directional modes and a plurality of directional modes. Thenon-directional mode may include, for example, a DC mode and a planarmode. The directional mode may include, for example, 33 directionalprediction modes or 65 directional prediction modes according to thedegree of detail of the prediction direction. However, this is merely anexample, more or less directional prediction modes may be used dependingon a setting. The intra prediction unit 185 may determine the predictionmode applied to the current block by using a prediction mode applied toa neighboring block.

The inter prediction unit 180 may derive a predicted block for thecurrent block based on a reference block (reference sample array)specified by a motion vector on a reference picture. In this case, inorder to reduce the amount of motion information transmitted in theinter prediction mode, the motion information may be predicted in unitsof blocks, subblocks, or samples based on correlation of motioninformation between the neighboring block and the current block. Themotion information may include a motion vector and a reference pictureindex. The motion information may further include inter predictiondirection (L0 prediction, L1 prediction, Bi prediction, etc.)information. In the case of inter prediction, the neighboring block mayinclude a spatial neighboring block present in the current picture and atemporal neighboring block present in the reference picture. Thereference picture including the reference block and the referencepicture including the temporal neighboring block may be the same ordifferent. The temporal neighboring block may be called a collocatedreference block, a co-located CU (colCU), and the like. The referencepicture including the temporal neighboring block may be called acollocated picture (colPic). For example, the inter prediction unit 180may configure a motion information candidate list based on neighboringblocks and generate information indicating which candidate is used toderive a motion vector and/or a reference picture index of the currentblock. Inter prediction may be performed based on various predictionmodes. For example, in the case of a skip mode and a merge mode, theinter prediction unit 180 may use motion information of the neighboringblock as motion information of the current block. In the case of theskip mode, unlike the merge mode, the residual signal may not betransmitted. In the case of the motion vector prediction (MVP) mode, themotion vector of the neighboring block may be used as a motion vectorpredictor, and the motion vector of the current block may be signaled byencoding a motion vector difference and an indicator for a motion vectorpredictor. The motion vector difference may mean a difference betweenthe motion vector of the current block and the motion vector predictor.

The prediction unit may generate a prediction signal based on variousprediction methods and prediction techniques described below. Forexample, the prediction unit may not only apply intra prediction orinter prediction but also simultaneously apply both intra prediction andinter prediction, in order to predict the current block. A predictionmethod of simultaneously applying both intra prediction and interprediction for prediction of the current block may be called combinedinter and intra prediction (CIIP). In addition, the prediction unit mayperform intra block copy (IBC) for prediction of the current block.Intra block copy may be used for content image/video coding of a game orthe like, for example, screen content coding (SCC). IBC is a method ofpredicting a current picture using a previously reconstructed referenceblock in the current picture at a location apart from the current blockby a predetermined distance. When IBC is applied, the location of thereference block in the current picture may be encoded as a vector (blockvector) corresponding to the predetermined distance. IBC basicallyperforms prediction in the current picture, but may be performedsimilarly to inter prediction in that a reference block is derivedwithin the current picture. That is, IBC may use at least one of theinter prediction techniques described in the present disclosure.

The prediction signal generated by the prediction unit may be used togenerate a reconstructed signal or to generate a residual signal. Thesubtractor 115 may generate a residual signal (residual block orresidual sample array) by subtracting the prediction signal (predictedblock or prediction sample array) output from the prediction unit fromthe input image signal (original block or original sample array). Thegenerated residual signal may be transmitted to the transformer 120.

The transformer 120 may generate transform coefficients by applying atransform technique to the residual signal. For example, the transformtechnique may include at least one of a discrete cosine transform (DCT),a discrete sine transform (DST), a karhunen-loève transform (KLT), agraph-based transform (GBT), or a conditionally non-linear transform(CNT). Here, the GBT means transform obtained from a graph whenrelationship information between pixels is represented by the graph. TheCNT refers to transform acquired based on a prediction signal generatedusing all previously reconstructed pixels. In addition, the transformprocess may be applied to square pixel blocks having the same size ormay be applied to blocks having a variable size rather than square.

The quantizer 130 may quantize the transform coefficients and transmitthem to the entropy encoder 190. The entropy encoder 190 may encode thequantized signal (information on the quantized transform coefficients)and output a bitstream. The information on the quantized transformcoefficients may be referred to as residual information. The quantizer130 may rearrange quantized transform coefficients in a block form intoa one-dimensional vector form based on a coefficient scanning order andgenerate information on the quantized transform coefficients based onthe quantized transform coefficients in the one-dimensional vector form.

The entropy encoder 190 may perform various encoding methods such as,for example, exponential Golomb, context-adaptive variable length coding(CAVLC), context-adaptive binary arithmetic coding (CABAC), and thelike. The entropy encoder 190 may encode information necessary forvideo/image reconstruction other than quantized transform coefficients(e.g., values of syntax elements, etc.) together or separately. Encodedinformation (e.g., encoded video/image information) may be transmittedor stored in units of network abstraction layers (NALs) in the form of abitstream. The video/image information may further include informationon various parameter sets such as an adaptation parameter set (APS), apicture parameter set (PPS), a sequence parameter set (SPS), or a videoparameter set (VPS). In addition, the video/image information mayfurther include general constraint information. The signaledinformation, transmitted information and/or syntax elements described inthe present disclosure may be encoded through the above-describedencoding procedure and included in the bitstream.

The bitstream may be transmitted over a network or may be stored in adigital storage medium. The network may include a broadcasting networkand/or a communication network, and the digital storage medium mayinclude various storage media such as USB, SD, CD, DVD, Blu-ray, HDD,SSD, and the like. A transmitter (not shown) transmitting a signaloutput from the entropy encoder 190 and/or a storage unit (not shown)storing the signal may be included as internal/external element of theimage encoding apparatus 100. Alternatively, the transmitter may beprovided as the component of the entropy encoder 190.

The quantized transform coefficients output from the quantizer 130 maybe used to generate a residual signal. For example, the residual signal(residual block or residual samples) may be reconstructed by applyingdequantization and inverse transform to the quantized transformcoefficients through the dequantizer 140 and the inverse transformer150.

The adder 155 adds the reconstructed residual signal to the predictionsignal output from the inter prediction unit 180 or the intra predictionunit 185 to generate a reconstructed signal (reconstructed picture,reconstructed block, reconstructed sample array). If there is noresidual for the block to be processed, such as a case where the skipmode is applied, the predicted block may be used as the reconstructedblock. The adder 155 may be called a reconstructor or a reconstructedblock generator. The generated reconstructed signal may be used forintra prediction of a next block to be processed in the current pictureand may be used for inter prediction of a next picture through filteringas described below.

The filter 160 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter160 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 170, specifically, a DPB of thememory 170. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like. The filter 160 may generate variousinformation related to filtering and transmit the generated informationto the entropy encoder 190 as described later in the description of eachfiltering method. The information related to filtering may be encoded bythe entropy encoder 190 and output in the form of a bitstream.

The modified reconstructed picture transmitted to the memory 170 may beused as the reference picture in the inter prediction unit 180. Wheninter prediction is applied through the image encoding apparatus 100,prediction mismatch between the image encoding apparatus 100 and theimage decoding apparatus may be avoided and encoding efficiency may beimproved.

The DPB of the memory 170 may store the modified reconstructed picturefor use as a reference picture in the inter prediction unit 180. Thememory 170 may store the motion information of the block from which themotion information in the current picture is derived (or encoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter prediction unit 180 and used as the motion information of thespatial neighboring block or the motion information of the temporalneighboring block. The memory 170 may store reconstructed samples ofreconstructed blocks in the current picture and may transfer thereconstructed samples to the intra prediction unit 185.

Overview of Image Decoding Apparatus

FIG. 3 is a view schematically showing an image decoding apparatus, towhich an embodiment of the present disclosure is applicable.

As shown in FIG. 3, the image decoding apparatus 200 may include anentropy decoder 210, a dequantizer 220, an inverse transformer 230, anadder 235, a filter 240, a memory 250, an inter prediction unit 260 andan intra prediction unit 265. The inter prediction unit 260 and theintra prediction unit 265 may be collectively referred to as a“prediction unit”. The dequantizer 220 and the inverse transformer 230may be included in a residual processor.

All or at least some of a plurality of components configuring the imagedecoding apparatus 200 may be configured by a hardware component (e.g.,a decoder or a processor) according to an embodiment. In addition, thememory 170 may include a decoded picture buffer (DPB) or may beconfigured by a digital storage medium.

The image decoding apparatus 200, which has received a bitstreamincluding video/image information, may reconstruct an image byperforming a process corresponding to a process performed by the imageencoding apparatus 100 of FIG. 2. For example, the image decodingapparatus 200 may perform decoding using a processing unit applied inthe image encoding apparatus. Thus, the processing unit of decoding maybe a coding unit, for example. The coding unit may be acquired bypartitioning a coding tree unit or a largest coding unit. Thereconstructed image signal decoded and output through the image decodingapparatus 200 may be reproduced through a reproducing apparatus (notshown).

The image decoding apparatus 200 may receive a signal output from theimage encoding apparatus of FIG. 2 in the form of a bitstream. Thereceived signal may be decoded through the entropy decoder 210. Forexample, the entropy decoder 210 may parse the bitstream to deriveinformation (e.g., video/image information) necessary for imagereconstruction (or picture reconstruction). The video/image informationmay further include information on various parameter sets such as anadaptation parameter set (APS), a picture parameter set (PPS), asequence parameter set (SPS), or a video parameter set (VPS). Inaddition, the video/image information may further include generalconstraint information. The image decoding apparatus may further decodepicture based on the information on the parameter set and/or the generalconstraint information. Signaled/received information and/or syntaxelements described in the present disclosure may be decoded through thedecoding procedure and obtained from the bitstream. For example, theentropy decoder 210 decodes the information in the bitstream based on acoding method such as exponential Golomb coding, CAVLC, or CABAC, andoutput values of syntax elements required for image reconstruction andquantized values of transform coefficients for residual. Morespecifically, the CABAC entropy decoding method may receive a bincorresponding to each syntax element in the bitstream, determine acontext model using a decoding target syntax element information,decoding information of a neighboring block and a decoding target blockor information of a symbol/bin decoded in a previous stage, and performarithmetic decoding on the bin by predicting a probability of occurrenceof a bin according to the determined context model, and generate asymbol corresponding to the value of each syntax element. In this case,the CABAC entropy decoding method may update the context model by usingthe information of the decoded symbol/bin for a context model of a nextsymbol/bin after determining the context model. The information relatedto the prediction among the information decoded by the entropy decoder210 may be provided to the prediction unit (the inter prediction unit260 and the intra prediction unit 265), and the residual value on whichthe entropy decoding was performed in the entropy decoder 210, that is,the quantized transform coefficients and related parameter information,may be input to the dequantizer 220. In addition, information onfiltering among information decoded by the entropy decoder 210 may beprovided to the filter 240. Meanwhile, a receiver (not shown) forreceiving a signal output from the image encoding apparatus may befurther configured as an internal/external element of the image decodingapparatus 200, or the receiver may be a component of the entropy decoder210.

Meanwhile, the image decoding apparatus according to the presentdisclosure may be referred to as a video/image/picture decodingapparatus. The image decoding apparatus may be classified into aninformation decoder (video/image/picture information decoder) and asample decoder (video/image/picture sample decoder). The informationdecoder may include the entropy decoder 210. The sample decoder mayinclude at least one of the dequantizer 220, the inverse transformer230, the adder 235, the filter 240, the memory 250, the inter predictionunit 160 or the intra prediction unit 265.

The dequantizer 220 may dequantize the quantized transform coefficientsand output the transform coefficients. The dequantizer 220 may rearrangethe quantized transform coefficients in the form of a two-dimensionalblock. In this case, the rearrangement may be performed based on thecoefficient scanning order performed in the image encoding apparatus.The dequantizer 220 may perform dequantization on the quantizedtransform coefficients by using a quantization parameter (e.g.,quantization step size information) and obtain transform coefficients.

The inverse transformer 230 may inversely transform the transformcoefficients to obtain a residual signal (residual block, residualsample array).

The prediction unit may perform prediction on the current block andgenerate a predicted block including prediction samples for the currentblock. The prediction unit may determine whether intra prediction orinter prediction is applied to the current block based on theinformation on the prediction output from the entropy decoder 210 andmay determine a specific intra/inter prediction mode (predictiontechnique).

It is the same as described in the prediction unit of the image encodingapparatus 100 that the prediction unit may generate the predictionsignal based on various prediction methods (techniques) which will bedescribed later.

The intra prediction unit 265 may predict the current block by referringto the samples in the current picture. The description of the intraprediction unit 185 is equally applied to the intra prediction unit 265.

The inter prediction unit 260 may derive a predicted block for thecurrent block based on a reference block (reference sample array)specified by a motion vector on a reference picture. In this case, inorder to reduce the amount of motion information transmitted in theinter prediction mode, motion information may be predicted in units ofblocks, subblocks, or samples based on correlation of motion informationbetween the neighboring block and the current block. The motioninformation may include a motion vector and a reference picture index.The motion information may further include inter prediction direction(L0 prediction, L1 prediction, Bi prediction, etc.) information. In thecase of inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. For example, theinter prediction unit 260 may configure a motion information candidatelist based on neighboring blocks and derive a motion vector of thecurrent block and/or a reference picture index based on the receivedcandidate selection information. Inter prediction may be performed basedon various prediction modes, and the information on the prediction mayinclude information indicating a mode of inter prediction for thecurrent block.

The adder 235 may generate a reconstructed signal (reconstructedpicture, reconstructed block, reconstructed sample array) by adding theobtained residual signal to the prediction signal (predicted block,predicted sample array) output from the prediction unit (including theinter prediction unit 260 and/or the intra prediction unit 265). Ifthere is no residual for the block to be processed, such as when theskip mode is applied, the predicted block may be used as thereconstructed block. The description of the adder 155 is equallyapplicable to the adder 235. The adder 235 may be called a reconstructoror a reconstructed block generator. The generated reconstructed signalmay be used for intra prediction of a next block to be processed in thecurrent picture and may be used for inter prediction of a next picturethrough filtering as described below.

The filter 240 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter240 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 250, specifically, a DPB of thememory 250. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 250may be used as a reference picture in the inter prediction unit 260. Thememory 250 may store the motion information of the block from which themotion information in the current picture is derived (or decoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter prediction unit 260 so as to be utilized as the motioninformation of the spatial neighboring block or the motion informationof the temporal neighboring block. The memory 250 may storereconstructed samples of reconstructed blocks in the current picture andtransfer the reconstructed samples to the intra prediction unit 265.

In the present disclosure, the embodiments described in the filter 160,the inter prediction unit 180, and the intra prediction unit 185 of theimage encoding apparatus 100 may be equally or correspondingly appliedto the filter 240, the inter prediction unit 260, and the intraprediction unit 265 of the image decoding apparatus 200.

Overview of Partitioning of CTU

As described above, the coding unit may be acquired by recursivelypartitioning the coding tree unit (CTU) or the largest coding unit (LCU)according to a quad-tree/binary-tree/ternary-tree (QT/BT/TT) structure.For example, the CTU may be first partitioned by quadtree structures.Thereafter, leaf nodes of the quadtree structure may be furtherpartitioned by a multi-type tree structure.

Partitioning according to quadtree means that a current CU (or CTU) ispartitioned into equally four. By partitioning according to quadtree,the current CU may be partitioned into four CUs having the same widthand the same height. When the current CU is no longer partitioned by thequadtree structure, the current CU corresponds to the leaf node of thequad-tree structure. The CU corresponding to the leaf node of thequadtree structure may be no longer partitioned and may be used as theabove-described final coding unit. Alternatively, the CU correspondingto the leaf node of the quadtree structure may be further partitioned bya multi-type tree structure.

FIG. 4 is a view showing an embodiment of a partitioning type of a blockaccording to a multi-type tree structure. Partitioning according to themulti-type tree structure may include two types of splitting accordingto a binary tree structure and two types of splitting according to aternary tree structure.

The two types of splitting according to the binary tree structure mayinclude vertical binary splitting (SPLIT_BT_VER) and horizontal binarysplitting (SPLIT_BT_HOR). Vertical binary splitting (SPLIT_BT_VER) meansthat the current CU is split into equally two in the vertical direction.As shown in FIG. 4, by vertical binary splitting, two CUs having thesame height as the current CU and having a width which is half the widthof the current CU may be generated. Horizontal binary splitting(SPLIT_BT_HOR) means that the current CU is split into equally two inthe horizontal direction. As shown in FIG. 4, by horizontal binarysplitting, two CUs having a height which is half the height of thecurrent CU and having the same width as the current CU may be generated.

Two types of splitting according to the ternary tree structure mayinclude vertical ternary splitting (SPLIT_TT_VER) and horizontal ternarysplitting (SPLIT_TT_HOR). In vertical ternary splitting (SPLIT_TT_VER),the current CU is split in the vertical direction at a ratio of 1:2:1.As shown in FIG. 4, by vertical ternary splitting, two CUs having thesame height as the current CU and having a width which is ¼ of the widthof the current CU and a CU having the same height as the current CU andhaving a width which is half the width of the current CU may begenerated. In horizontal ternary splitting (SPLIT_TT_HOR), the currentCU is split in the horizontal direction at a ratio of 1:2:1. As shown inFIG. 4, by horizontal ternary splitting, two CUs having a height whichis ¼ of the height of the current CU and having the same width as thecurrent CU and a CU having a height which is half the height of thecurrent CU and having the same width as the current CU may be generated.

FIG. 5 is a view showing a signaling mechanism of partition splittinginformation in a quadtree with nested multi-type tree structureaccording to the present disclosure.

Here, the CTU is treated as the root node of the quadtree, and ispartitioned for the first time by a quadtree structure. Information(e.g., qt_split_flag) indicating whether quadtree splitting is performedwith respect to the current CU (CTU or node (QT_node) of the quadtree)may be signaled. For example, when qt_split_flag has a first value(e.g., “1”), the current CU may be quadtree-partitioned. In addition,when qt_split_flag has a second value (e.g., “0”), the current CU is notquadtree-partitioned, but becomes the leaf node (QT_leaf_node) of thequadtree. Each quadtree leaf node may then be further partitioned intomultitype tree structures. That is, the leaf node of the quadtree maybecome the node (MTT_node) of the multi-type tree. In the multitype treestructure, a first flag (e.g., mtt_split_cu_flag) is signaled toindicate whether the current node is additionally partitioned. If thecorresponding node is additionally partitioned (e.g., if the first flagis 1), a second flag (e.g., mtt_split_cu_vertical_flag) may be signaledto indicate the splitting direction. For example, the splittingdirection may be a vertical direction if the second flag is 1 and may bea horizontal direction if the second flag is 0. Then, a third flag(e.g., mtt_split_cu_binary_flag) may be signaled to indicate whether thesplit type is a binary split type or a ternary split type. For example,the split type may be a binary split type when the third flag is 1 andmay be a ternary split type when the third flag is 0. The node of themulti-type tree acquired by binary splitting or ternary splitting may befurther partitioned by multi-type tree structures. However, the node ofthe multi-type tree may not be partitioned by quadtree structures. Ifthe first flag is 0, the corresponding node of the multi-type tree is nolonger split but becomes the leaf node (MTT_leaf_node) of the multi-typetree. The CU corresponding to the leaf node of the multi-type tree maybe used as the above-described final coding unit.

Based on the mtt_split_cu_vertical_flag and themtt_split_cu_binary_flag, a multi-type tree splitting mode(MttSplitMode) of a CU may be derived as shown in Table 1 below.

TABLE 1 MttSplitMode mtt_split_cu_vertical_flag mtt_split_cu_binary_flagSPLIT_TT_HOR 0 0 SPLIT_BT_HOR 0 1 SPLIT_TT_VER 1 0 SPLIT_BT_VER 1 1

One CTU may include a coding block of luma samples (hereinafter referredto as a “luma block”) and two coding blocks of chroma samplescorresponding thereto (hereinafter referred to as “chroma blocks”). Theabove-described coding tree scheme may be equally or separately appliedto the luma block and chroma block of the current CU. Specifically, theluma and chroma blocks in one CTU may be partitioned into the same blocktree structure and, in this case, the tree structure may be representedas SINGLE_TREE. Alternatively, the luma and chroma blocks in one CTU maybe partitioned into separate block tree structures, and, in this case,the tree structure may be represented as DUAL_TREE. That is, when theCTU is partitioned into dual trees, the block tree structure for theluma block and the block tree structure for the chroma block may beseparately present. In this case, the block tree structure for the lumablock may be called DUAL_TREE_LUMA, and the block tree structure for thechroma component may be called DUAL_TREE_CHROMA. For P and B slice/tilegroups, luma and chroma blocks in one CTU may be limited to have thesame coding tree structure. However, for I slice/tile groups, luma andchroma blocks may have a separate block tree structure from each other.If the separate block tree structure is applied, the luma CTB may bepartitioned into CUs based on a particular coding tree structure, andthe chroma CTB may be partitioned into chroma CUs based on anothercoding tree structure. That is, a CU in an I slice/tile group, to whichthe individual block tree structure applies, may include a coding blockof luma components or coding blocks of two chroma components. Inaddition, a CU in an I slice/tile group, to which the same block treestructure applies, and a CU of a P or B slice/tile group may includeblocks of three color components (a luma component and two chromacomponents). Although a quadtree coding tree structure with a nestedmultitype tree has been described, a structure in which a CU ispartitioned is not limited thereto. For example, the BT structure andthe TT structure may be interpreted as a concept included in a multiplepartitioning tree (MPT) structure, and the CU may be interpreted asbeing partitioned through the QT structure and the MPT structure. In anexample where the CU is partitioned through a QT structure and an MPTstructure, a syntax element (e.g., MPT_split_type) including informationon how many blocks the leaf node of the QT structure is partitioned intoand a syntax element (ex. MPT_split_mode) including information on whichof vertical and horizontal directions the leaf node of the QT structureis partitioned into may be signaled to determine a partitioningstructure.

In another example, a CU may be partitioned in a way different from a QTstructure, a BT structure or a TT structure. That is, unlikepartitioning a CU of a lower depth into a size of ¼ of a CU of a higherdepth according to the QT size, partitioning a CU of a lower depth intoa size of ½ of a CU of a higher depth according to the BT size orpartitioning a CU of a lower depth into a size of ¼ or ½ of a CU of ahigher depth according to the TT size, the CU of the lower depth may bepartitioned into a size of ⅕, ⅓, ⅜, ⅗, ⅔ or ⅝ of a CU of a higher depth,and a method of partitioning a CU is not limited thereto.

Overview of Intra Prediction

Hereinafter, intra prediction according to an embodiment will bedescribed.

Intra prediction may indicate prediction which generates predictionsamples for a current block based on reference samples in a picture towhich the current block belongs (hereinafter referred to as a currentpicture). When intra prediction applies to the current block,neighboring reference samples to be used for intra prediction of thecurrent block may be derived. The neighboring reference samples of thecurrent block may include a sample adjacent to a left boundary of thecurrent block having a size of nWxnH and a total of 2xnH samplesadjacent to the bottom-left, a sample adjacent to a top boundary of thecurrent block and a total of 2xnW samples adjacent to the top-right, andone sample adjacent to the top-left of the current block. Alternatively,the neighboring reference samples of the current block may include aplurality of columns of top neighboring samples and a plurality of rowsof left neighboring samples. In addition, the neighboring referencesamples of the current block may include a total of nH samples adjacentto a right boundary of the current block having a size of nWxnH, a totalof nW samples adjacent to a bottom boundary of the current block, andone sample adjacent to the bottom-right of the current block.

Some of the neighboring reference samples of the current block have notyet been decoded or may not be available. In this case, a decoder mayconstruct neighboring reference samples to be used for prediction, bysubstituting unavailable samples with available samples. Alternatively,neighboring reference samples to be used for prediction may beconstructed using interpolation of available samples.

When the neighboring reference samples are derived, (i) a predictionsample may be derived based on average or interpolation of neighboringreference samples of the current block and (ii) the prediction samplemay be derived based on a reference sample present in a specific(prediction) direction with respect to the prediction sample among theneighboring reference samples of the current block. The case of (i) maybe referred to as a non-directional mode or a non-angular mode and thecase of (ii) may be referred to as a directional mode or an angularmode.

In addition, the prediction sample may be generated throughinterpolation with a first neighboring sample located in a predictiondirection of the intra prediction mode of the current block and a secondneighboring sample located in the opposite direction based on aprediction target sample of the current block among the neighboringreference samples. The above-described case may be referred to as linearinterpolation intra prediction (LIP).

In addition, chroma prediction samples may be generated based on lumasamples using a linear model. This case may be called a linear model(LM) mode.

In addition, a temporary prediction sample of the current block may bederived based on filtered neighboring reference samples, and theprediction sample of the current block may be derived byweighted-summing the temporary prediction sample and at least onereference sample derived according to the intra prediction mode amongthe existing neighboring reference samples, that is, the unfilteredneighboring reference samples. This case may be referred to as positiondependent intra prediction (PDPC).

In addition, a reference sample line with highest prediction accuracymay be selected from multiple neighboring reference sample lines of thecurrent block to derive a prediction sample using a reference samplelocated in a prediction direction in the corresponding line, and, atthis time, information (e.g., intra_luma_ref_idx) on the used referencesample line may be encoded and signaled in a bitstream. This case may bereferred to as multi-reference line (MRL) intra prediction or MRL basedintra prediction. When MRL does not apply, reference samples may bederived from a reference sample line directly adjacent to the currentblock. In this case, information on the reference sample line may not besignaled.

In addition, the current block may be split into vertical or horizontalsub-partitions to perform intra prediction with respect to eachsub-partition based on the same intra prediction mode. At this time,neighboring reference samples of intra prediction may be derived inunits of sub-partitions. That is, a reconstructed sample of a previoussub-partition in encoding/decoding order may be used as a neighboringreference sample of a current sub-partition. In this case, the intraprediction mode for the current block equally applies to thesub-partitions and the neighboring reference samples are derived andused in units of sub-partitions, thereby increasing intra predictionperformance. Such a prediction method may be referred to as intrasub-partitions (ISP) or ISP based intra prediction.

The intra prediction technique may be referred to as various terms suchas intra prediction type or additional intra prediction mode to bedistinguished from a directional or non-directional intra predictionmode. For example, the intra prediction technique (intra prediction typeor the additional intra prediction mode) may include at least one ofLIP, LM, PDPC, MRL, ISP or MIP. A general intra prediction methodexcluding a specific intra prediction type such as LIP, LM, PDPC, MRL orISP may be referred to as a normal intra prediction type. The normalintra prediction type is generally applicable when the above-describedspecific intra prediction type does not apply, and prediction may beperformed based on the above-described intra prediction mode. Meanwhile,if necessary, post-filtering may be performed with respect to thederived prediction sample.

Specifically, the intra prediction procedure may include an intraprediction mode/type determination step, a neighboring reference samplederivation step and an intra prediction mode/type based predictionsample derivation step. In addition, if necessary, post-filtering may beperformed with respect to the derived prediction sample.

FIG. 6 is a flowchart illustrating an intra prediction based video/imageencoding method.

The encoding method of FIG. 6 may be performed by the image encodingapparatus of FIG. 2. Specifically, step S610 may be performed by theintra prediction unit 185, and step S620 may be performed by theresidual processor. Specifically, step S620 may be performed by thesubtractor 115. Step S630 may be performed by the entropy encoder 190.The prediction information of step S630 may be derived by the intraprediction unit 185, and the residual information of step S630 may bederived by the residual processor. The residual information isinformation on the residual samples. The residual information mayinclude information on quantized transform coefficient for the residualsamples. As described above, the residual samples may be derived astransform coefficient through the transformer 120 of the image encodingapparatus, and the transform coefficient may be derived as the transformcoefficients quantized through the quantizer 130. The information on thequantized transform coefficients may be encoded by the entropy encoder190 through a residual coding procedure.

The image encoding apparatus may perform intra prediction with respectto a current block (S610). The image encoding apparatus may determine anintra prediction mode/type for the current block, derive neighboringreference samples of the current block, and generate prediction samplesin the current block based on the intra prediction mode/type and theneighboring reference samples. Here, the intra prediction mode/typedetermination, neighboring reference samples derivation and predictionsamples generation procedures may be simultaneously performed or any oneprocedure may be performed before the other procedures.

FIG. 7 is a view illustrating the configuration of an intra predictionunit 185 according to the present disclosure.

As shown in FIG. 7, the intra prediction unit 185 of the image encodingapparatus may include an intra prediction mode/type determination unit186, a reference sample derivation unit 187 and/or a prediction samplederivation unit 188. The intra prediction mode/type determination unit186 may determine an intra prediction mode/type for the current block.The reference sample derivation unit 187 may derive neighboringreference samples of the current block. The prediction sample derivationunit 188 may derive prediction samples of the current block. Meanwhile,although not shown, when the below-described prediction sample filteringprocedure is performed, the intra prediction unit 185 may furtherinclude a prediction sample filter (not shown).

The image encoding apparatus may determine a mode/type applying to thecurrent block among a plurality of intra prediction modes/types. Theimage encoding apparatus may compare rate distortion (RD) cost for theintra prediction modes/types and determine an optimal intra predictionmode/type for the current block.

Meanwhile, the image encoding apparatus may perform a prediction samplefiltering procedure. Prediction sample filtering may be referred to aspost-filtering. By the prediction sample filtering procedure, some orall of the prediction samples may be filtered. In some cases, theprediction sample filtering procedure may be omitted.

Referring to FIG. 6 again, the image encoding apparatus may generateresidual samples for the current block based on the prediction samplesor the filtered prediction samples (S620). The image encoding apparatusmay derive the residual samples by subtracting the prediction samplesfrom the original samples of the current block. That is, the imageencoding apparatus may derive the residual sample values by subtractingthe corresponding prediction sample value from the original samplevalue.

The image encoding apparatus may encode image information includinginformation on the intra prediction (prediction information) andresidual information of the residual samples (S630). The predictioninformation may include the intra prediction mode information and/or theintra prediction technique information. The image encoding apparatus mayoutput the encoded image information in the form of a bitstream. Theoutput bitstream may be transmitted to the image decoding apparatusthrough a storage medium or a network.

The residual information may include residual coding syntax, which willbe described later. The image encoding apparatus may transform/quantizethe residual samples and derive quantized transform coefficients. Theresidual information may include information on the quantized transformcoefficients.

Meanwhile, as described above, the image encoding apparatus may generatea reconstructed picture (including reconstructed samples and areconstructed block). To this end, the image encoding apparatus mayperform dequantization/inverse transform with respect to the quantizedtransform coefficients and derive (modified) residual samples. Thereason for transforming/quantizing the residual samples and thenperforming dequantization/inverse transform is to derive the sameresidual samples as residual samples derived by the image decodingapparatus. The image encoding apparatus may generate a reconstructedbock including reconstructed samples for the current block based on theprediction samples and the (modified) residual samples. Based on thereconstructed block, the reconstructed picture for the current picturemay be generated. As described above, an in-loop filtering procedure isfurther applicable to the reconstructed picture.

FIG. 8 is a flowchart illustrating an intra prediction based video/imagedecoding method.

The image decoding apparatus may perform operation corresponding tooperation performed by the image encoding apparatus.

The decoding method of FIG. 8 may be performed by the image decodingapparatus of FIG. 3. Steps S810 to S830 may be performed by the intraprediction unit 265, and the prediction information of step S810 and theresidual information of step S840 may be obtained from a bitstream bythe entropy decoder 210. The residual processor of the image decodingapparatus may derive residual samples for the current block based on theresidual information (S840). Specifically, the dequantizer 220 of theresidual processor may perform dequantization based on the dequantizedtransform coefficients derived based on the residual information toderive transform coefficients, and the inverse transformer 230 of theresidual processor may perform inverse transform with respect to thetransform coefficients to derive the residual samples for the currentblock. Step S850 may be performed by the adder 235 or the reconstructor.

Specifically, the image decoding apparatus may derive an intraprediction mode/type for the current block based on the receivedprediction information (intra prediction mode/type information) (S810).The image decoding apparatus may derive neighboring reference samples ofthe current block (S820). The image decoding apparatus may generateprediction samples in the current block based on the intra predictionmode/type and the neighboring reference samples (S830). In this case,the image decoding apparatus may perform a prediction sample filteringprocedure. Prediction sample filtering may be referred to aspost-filtering. By the prediction sample filtering procedure, some orall of the prediction samples may be filtered. In some cases, theprediction sample filtering procedure may be omitted.

The image decoding apparatus may generate residual samples for thecurrent block based on the received residual information (S840). Theimage decoding apparatus may generate reconstructed samples for thecurrent block based on the prediction samples and the residual samplesand derive a reconstructed block including the reconstructed samples(S850). Based on the reconstructed block, the reconstructed picture forthe current picture may be generated. An in-loop filtering procedure isfurther applicable to the reconstructed picture, as described above.

FIG. 9 is a view illustrating the configuration of an intra predictionunit 265 according to the present disclosure.

As shown in FIG. 9, the intra prediction unit 265 of the image decodingapparatus may include an intra prediction mode/type determination unit266, a reference sample derivation unit 267 and a prediction samplederivation unit 268. The intra prediction mode/type determination unit266 may determine an intra prediction mode/type for the current blockbased on the intra prediction mode/type information generated andsignaled by the intra prediction mode/type determination unit 186 of theimage encoding apparatus, and the reference sample derivation unit 267may derive neighboring reference samples of the current block from areconstructed reference region in a current picture. The predictionsample derivation unit 268 may derive prediction samples of the currentblock. Meanwhile, although not shown, when the above-describedprediction sample filtering procedure is performed, the intra predictionunit 265 may further include a prediction sample filter (not shown).

The intra prediction mode information may include, for example, flaginformation (e.g., intra_luma_mpm_flag) indicating whether to apply amost probable mode (MPM) or a remaining mode to the current block, and,when the MPM applies to the current block, the intra prediction modeinformation may further include index information (e.g.,intra_luma_mpm_idx) indicating one of the intra prediction modecandidates (MPM candidates). The intra prediction mode candidates (MPMcandidates) may be composed of an MPM candidate list or an MPM list. Inaddition, when the MPM does not apply to the current block, the intraprediction mode information may further include remaining modeinformation (e.g., intra_luma_mpm_remainder) indicating one of theremaining intra prediction modes excluding the intra prediction modecandidates (MPM candidates). The image decoding apparatus may determinethe intra prediction mode of the current block based on the intraprediction mode information. The MPM candidate modes may include theintra prediction modes of the neighboring blocks (e.g., the leftneighboring block and the upper neighboring block) of the current blockand additional candidate modes.

In addition, the intra prediction technique information may beimplemented in various forms. For example, the intra predictiontechnique information may include intra prediction technique indexinformation indicating one of the intra prediction techniques. Asanother example, the intra prediction technique information may includeat least one of reference sample line information (e.g.,intra_luma_ref_idx) indicating whether to apply MRL to the current blockand, if applied, which reference sample line is used, ISP flaginformation (e.g., intra_subpartitions_mode_flag) indicating whether toapply ISP to the current block, ISP type information (e.g.,intra_subpartitions_split_flag) indicating the split type ofsub-partitions when applying ISP, flag information indicating whether toapply PDPC, or flag information indicating whether to apply LIP. In thepresent disclosure, ISP flag information may be referred to as an ISPapplication indicator.

The intra prediction mode information and/or the intra predictiontechnique information may be encoded/decoded through the coding methoddescribed in the present disclosure. For example, the intra predictionmode information and/or the intra prediction technique information maybe encoded/decoded through entropy coding (e.g., CABAC, CAVLC) based ona truncated (rice) binary code.

Hereinafter, an intra prediction mode/type determination methodaccording to the present disclosure will be described in greater detail.

When applying intra prediction to a current block, an intra predictionmode applying to the current block may be determined using an intraprediction mode of a neighboring block. For example, an image decodingapparatus may construct a most probable mode (mpm) list derived based onan intra prediction mode of a neighboring block (e.g., a left and/or topneighboring block) of the current block and additional candidate modesand select one of mpm candidates in the mpm list based on a received mpmindex. Alternatively, the image decoding apparatus may select one of theremaining intra prediction modes which are not included in the mpm listbased on remaining intra prediction mode information. For example,whether the intra prediction mode applying to the current bock is in thempm candidates (that is, in the mpm list) or in the remaining mode maybe indicated based on an mpm flag (e.g., intra_luma_mpm_flag). A value 1of the mpm flag may indicate that the intra prediction mode for thecurrent block is in the mpm candidates (mpm list) and a value 0 of thempm flag may indicate that the intra prediction mode of the currentblock is not in the mpm candidates (mpm list). The mpm index may besignaled in the form of a syntax element mpm_idx or intra_luma_mpm_idx,and the remaining intra prediction mode information may be signaled inthe form of a syntax element rem_intra_luma_pred_mode_orintra_luma_mpm_remainder. For example, the remaining intra predictionmode information may indicate one of the remaining intra predictionmodes which are not included in the mpm candidates (mpm list) among allintra prediction modes and are indexed in order of prediction modenumbers. The intra prediction mode may be an intra prediction mode of aluma component (sample). Hereinafter, the intra prediction modeinformation may include at least one of the mpm flag (e.g.,intra_luma_mpm_flag), the mpm index (e.g., mpm_idx orintra_luma_mpm_idx) or the remaining intra prediction mode information(rem_intra_luma_pred_mode_or intra_luma_mpm_remainder). In the presentdisclosure, the MPM list may be referred to as various terms such as MPMcandidate list, candModeList, etc.

FIG. 10 is a flowchart illustrating an intra prediction mode signalingprocedure in an image encoding apparatus.

Referring to FIG. 10, the image encoding apparatus may construct an MPMlist for a current block (S1010). The MPM list may include candidateintra prediction modes (MPM candidates) which are highly likely to applyto the current block. The MPM list may include the intra prediction modeof a neighboring block and may further include specific intra predictionmodes according to a predetermined method.

The image encoding apparatus may determine the intra prediction mode ofthe current block (S1020). The image encoding apparatus may performprediction based on various intra prediction modes and performrate-distortion optimization (RDO) based on this to determine an optimalintra prediction mode. In this case, the image encoding apparatus maydetermine the optimal intra prediction mode using only MPM candidatesincluded in the MPM list or determine the optimal intra prediction modeby further using not only the MPM candidates included in the MPM listbut also the remaining intra prediction modes. Specifically, forexample, if the intra prediction type of the current block is a specifictype (e.g., LIP, MRL or ISP) which is not a normal intra predictiontype, the image encoding apparatus may determine the optimal intraprediction mode using only the MPM candidates. That is, in this case,the intra prediction mode of the current block may be determined onlyfrom the MPM candidates and, in this case, the mpm flag may not beencoded/signaled. The image decoding apparatus may estimate that the mpmflag is 1, without separately receiving the mpm flag, in the case of thespecific type.

Meanwhile, in general, when the intra prediction mode of the current isone of the MPM candidates in the MPM list, the image encoding apparatusmay generate an mpm index indicating one of the MPM candidates. If theintra prediction mode of the current block is not present in the MPMlist, remaining intra prediction mode information indicating the samemode as the intra prediction mode of the current among the remainingintra prediction modes which are not included in the MPM list may begenerated.

The image encoding apparatus may encode and output intra prediction modeinformation in the form of a bitstream (S1030). The intra predictionmode information may include the above-described mpm flag, mpm indexand/or remaining intra prediction mode information. In general, the mpmindex and the remaining intra prediction mode information have analternative relationship and thus are not simultaneously signaled inindicating the intra prediction mode of one block. That is, when thevalue of the mpm flag is 1, the mpm index may be signaled and, when thevalue of the mpm flag is 0, the remaining intra prediction modeinformation may be signaled. However, as described above, when applyinga specific intra prediction type to the current block, the mpm flag isnot signaled and the value thereof is inferred to be 1 and only the mpmindex may be signaled. That is, in this case, the intra prediction modeinformation may include only the mpm index.

Although, in the example shown in FIG. 10, S1020 is shown as beingperformed after S1010, this is an example and S1020 may be performedbefore S1010 or they may be simultaneously performed.

FIG. 11 is a flowchart illustrating an intra prediction modedetermination procedure in an image decoding apparatus.

The image decoding apparatus may determine an intra prediction mode of acurrent block based on the intra prediction mode information determinedand signaled by the image encoding apparatus.

Referring to FIG. 11, the image decoding apparatus may obtain intraprediction mode information from a bitstream (S1110). As describedabove, the intra prediction mode information may include at least one ofan mpm flag, an mpm index or a remaining intra prediction mode.

The image decoding apparatus may construct an MPM list (S1120). The MPMlist may be constructed to be equal to the MPM list constructed by theimage encoding apparatus. That is, the MPM list may include an intraprediction mode of a neighboring block and may further include specificintra prediction modes according to a predetermined method.

Although, in the example shown in FIG. 11, S1120 is shown as beingperformed after S1110, this is an example and S1120 may be performedbefore S1110 or they may be simultaneously performed.

The image decoding apparatus determines the intra prediction mode of thecurrent block based on the MPM list and the intra prediction modeinformation (S1130). Step S1130 will be described in greaer detail withreference to FIG. 12.

FIG. 12 is a flowchart illustrating an intra prediction mode derivationprocedure in more detail.

Steps S1210 and S1220 of FIG. 12 may correspond to steps S1110 and S1120of FIG. 11, respectively. Accordingly, a detailed description of stepsS1210 and S1220 will be omitted.

The image decoding apparatus may obtain intra prediction modeinformation from a bitstream, construct an MPM list (S1210 and S1220),and determine a predetermined condition (S1230). Specifically, as shownin FIG. 12, when the value of an mpm flag is 1 (in S1230, Yes), theimage decoding apparatus may derive a candidate indicated by the mpmindex among the MPM candidates in the MPM list as the intra predictionmode of the current block (S1240). As another example, when the value ofthe mpm flag is 0 (in S1230, No), the image decoding apparatus mayderive an intra prediction mode indicated by the remaining intraprediction mode information among remaining intra prediction modes whichare not included in the MPM list as the intra prediction mode of thecurrent block (S1250). Meanwhile, as another example, when the intraprediction type of the current block is a specific type (e.g., LIP, MRLor ISP) (in S1230, Yes), the image decoding apparatus may derive acandidate indicated by the mpm index in the MPM list as the intraprediction mode of the current block, without checking the mpm flag(S1240).

FIG. 13 is a view showing an intra prediction direction according to anembodiment of the present disclosure.

The intra prediction mode may include, for example, two non-directionalintra prediction modes and 33 directional intra prediction modes. Thenon-directional intra prediction modes may include a planar intraprediction mode and a DC intra prediction mode, and the directionalintra prediction modes may include second to 34^(th) intra predictionmodes. The planar intra prediction mode may be referred to as a planarmode, and the DC intra prediction mode may be referred to as a DC mode.

In order to capture any edge direction presented in natural video, asshown in FIG. 13, the intra prediction mode may include twonon-directional intra prediction modes and 65 extended directional intraprediction modes. The non-directional intra prediction modes may includea planar mode and a DC mode, and the directional intra prediction modesmay include second to 66^(th) intra prediction modes. The extended intraprediction modes are applicable to blocks having all sizes, and areapplicable to both a luma component (luma block) and a chroma component(chroma block).

Alternatively, the intra prediction mode may include two non-directionalintra prediction modes and 129 directional intra prediction modes. Thenon-directional intra prediction modes may include a planar mode and aDC mode, and the directional intra prediction modes may include secondto 130^(th) intra prediction modes.

Meanwhile, the intra prediction mode may further include across-component linear model (CCLM) mode for chroma samples in additionto the above-described intra prediction modes. The CCLM mode may besplit into L_CCLM, T_CCLM, LT_CCLM according to whether left samples,upper samples or both thereof is considered for LM parameter derivationand may apply only to a chroma component.

For example, the intra prediction mode may be, for example, indexed asshown in Table 2 below.

TABLE 2 Intra prediction mode Associated name 0 INTRA_PLANAR 1 INTRA_DC 2 . . . 66 INTRA_ANGULAR2 . . . INTRA_ANGULAR66 81 . . . 83INTRA_LT_CCLM, INTRA_L_CCLM, INTRA_T_CCLM

FIG. 14 shows an intra prediction direction according to anotherembodiment of the present disclosure. In FIG. 14, a dotted-linedirection shows a wide angle mode applying only to a non-square block.As shown in FIG. 14, in order to capture any edge direction presented innatural video, the intra prediction mode according to an embodiment mayinclude two non-directional intra prediction modes and 93 directionalintra prediction modes. The non-directional intra prediction modes mayinclude a planar mode and a DC mode, and the directional intraprediction modes may include second to 80^(th) and −1^(st) to −14^(th)intra prediction modes, as denoted by arrow of FIG. 14. The planar modemay be denoted by INTRA PLANAR, and the DC mode may be denoted byINTRA_DC. In addition, the directional intra prediction mode may bedenoted by INTRA_ANGULAR-14 to INTRA_ANGULAR-1 and INTRA_ANGULAR2 toINTRA_ANGULAR80. Hereinafter, a prediction sample derivation method of achroma component block according to the present disclosure will bedescribed in detail.

When intra prediction is performed with respect to a current block,prediction for a luma component block (luma block) of the current blockand prediction for a chroma component block (chroma block) areperformed, and, in this case, an intra prediction mode for the chromacomponent (chroma block) may be set separately from an intra predictionmode for the luma component (luma block).

For example, the intra prediction mode for the chroma block (intrachroma prediction mode) may be indicated based on intra chromaprediction mode information, and the intra chroma prediction modeinformation may be signaled in the form of a syntax elementintra_chroma_pred_mode. For example, the intra chroma prediction modeinformation may indicate a planar mode, a DC mode, a vertical mode, ahorizontal mode, a derived mode (DM), or CCM modes. For example, whenthe intra prediction mode includes two non-directional intra predictionmodes and 33 directional intra prediction modes, the planar mode mayindicate intra prediction mode 0, the DC mode may indicate intraprediction mode 1, the vertical mode may indicate intra prediction mode26, and the horizontal mode may indicate intra prediction mode 10. Asanother example, when the intra prediction mode includes twonon-directional intra prediction modes and 65 directional intraprediction modes, the planar mode may indicate intra prediction mode 0,the DC mode may indicate intra prediction mode 1, the vertical mode mayindicate intra prediction mode 50, and the horizontal mode may indicateintra prediction mode 18. DM may also be referred to as a direct mode.CCLM may also be referred to as LM.

Meanwhile, DM and CCLM are dependent intra prediction modes forpredicting the chroma block using information on the luma block. The DMmay indicate a mode in which the same intra prediction mode as the intraprediction mode for the luma component (luma block) applies as the intraprediction mode for the chroma component (chroma block). In addition,the CCLM may indicate an intra prediction mode in which samples derivedby subsampling reconstructed samples of the luma block in a process ofgenerating the prediction block of the chroma block and then applyingCCLM parameters α and β to the subsampled samples are used as theprediction samples of the chroma block.

Overview of DM Mode

When a current chroma block is predicted in the DM, the intra predictionmode of the current chroma block may be derived as the intra predictionmode of a corresponding luma block. For example, the intra predictionmode at a predetermined position of the corresponding luma block may beused as the intra prediction mode of the current chroma block.

FIG. 15 is a view showing a predetermined position for deriving an intraprediction mode of a current chroma block in a DM mode.

In FIG. 15, for example, a chroma block may be vertically binary-splitand a current chroma block may be a block of a shaded area. In thiscase, a luma block corresponding to the current chroma block may be ablock of a shaded area in the luma block.

In FIG. 15, the intra prediction mode (intra chroma prediction mode) ofthe current chroma block in the DM may be derived as an intra predictionmode (intra luma prediction mode) at a predetermined position in acorresponding luma block. For example, an intra prediction mode of ablock covering a center bottom-right sample (center position) (CR) inthe corresponding luma block may be determined to be the intraprediction mode of the current chroma block. However, the predeterminedposition is not limited to the center position and may be, for example,another position within the corresponding luma block such as a top-leftposition TL.

Alternatively, multiple direct modes (MDM) may apply to the currentchroma block.

Multiple DM is a mode used by extending the single DM to a plurality ofmodes. In order to derive the intra prediction mode of the currentchroma block, a DM candidate list including a plurality of DM modes maybe constructed and one of candidates included in the DM candidate listmay be derived as the intra prediction mode of the current chroma mode.When applying multiple DM, the DM candidate list may include a pluralityof DM candidates which will be described below.

-   -   intra prediction mode at CR, TL, TR, BL or BR position of the        corresponding luma block    -   intra prediction mode of L, A, BL, AR or AL position which is        the neighboring block of the current chroma block    -   PLANAR mode and DC mode    -   directional mode derived by adding or subtracting an offset        (e.g., 1) to or from an already included directional mode    -   default DM candidate mode: vertical mode, horizontal mode, modes        2, 34, 66, 10 and 26 (in the case of 65 directional modes)    -   when four default DM candidates (PLANAR mode, DC mode, vertical        mode and horizontal mode) are not included in the DM candidate        list, DM candidates which are already included are replaced with        default DM candidates which are not included.

Overview of Signaling of Intra Prediction Mode of Chroma Block

The intra prediction mode of the chroma block may be encoded using atotal of eight intra prediction modes. The eight intra prediction modesmay include 5 conventional intra prediction modes and cross-componentlinear model mode (CCLM) mode(s).

Whether or not CCLM is available may be determined based on informationsignaled at a higher level (e.g., sps_cclm_enabled_flag transmitted at asequence level). Table 3 shows a mapping table for intra prediction modederivation of the current chroma block when CCLM is not available(sps_cclm_enabled_flag=0).

TABLE 3 IntraPredModeY[ xCb + cbWidth / 2 ][ yCb + cbHeight / 2 ]intra_chroma_pred_mode[ xCb ][ yCb ] 0 50 18 1 X ( 0 <= X <= 66 ) 0 66 00 0 0 1 50 66 50 50 50 2 18 18 66 18 18 3 1 1 1 66 1 4 0 50 18 1 X

As shown in Table 3, the intra prediction mode of the chroma block maybe derived based on intra chroma prediction mode information(intra_chroma_pred_mode) and/or the intra prediction mode(IntraPredModeY) of the corresponding luma block. The intra predictionmode of the corresponding luma block may be determined to be the intraprediction mode of the luma block covering the center bottom-rightsample (center position) of the current block or the chroma block. Thecenter bottom-right sample position may be derived as (xCb+cbWidth/2,yCb+cbHeight/2), and, in this case, (xCb, yCb) means the coordinates ofthe top-left sample of the corresponding luma block, and cbWidth andcbHeight respectively means the width and height of the correspondingluma block. For example, in Table 4, when intra_chroma_pred_mode is 0,the intra prediction mode of the chroma block is determined to be 0(planar mode), and, when intra_chroma_pred_mode is 1, the intraprediction mode of the chroma block may be determined to be 50 (verticalmode). In addition, when intra_chroma_pred_mode is 2, the intraprediction mode of the chroma block may be determined to be 18(horizontal mode), and, when intra_chroma_pred_mode is 3, the intraprediction mode of the chroma block may be determined to be 1 (DC mode).In addition, when intra_chroma_pred_mode is 4, the intra prediction modeof the chroma block may be determined to be the same value as the intraprediction mode of the corresponding luma block. That is, whenintra_chroma_pred_mode being 4 indicates that the intra prediction modeof the chroma block is derived as DM. The indices of the intraprediction modes of the chroma block (IntraPredModeC[xCb] [yCb]) derivedbased on Table 3 may correspond to the indices of the intra predictionmodes shown in Table 2.

According to Table 4, when the value of intra_chroma_pred_mode is 0 to3, the intra prediction mode of the chroma block may be determined to be66 instead of the above-described intra prediction mode (planar mode,vertical mode, horizontal mode or DC mode) according to the value ofIntraPredModeY. For example, the value 0 of intra_chroma_pred_modeindicates a planar mode, and, in this case, when IntraPredModeY is 0(planar mode), the intra prediction mode of the chroma block may bedetermined to be 66. However, as described above, when both the intraprediction mode of the chroma block and the intra prediction mode of thecorresponding luma block are the same as the planar mode, the value ofintra_chroma_pred_mode may be determined to be 4 which indicates the DMinstead of 0. Accordingly, when the value of intra_chroma_pred_mode is 0to 3, the case where the intra prediction mode of the chroma block isdetermined to be 66 does not substantially occur. That is, whenintra_chroma_pred_mode is 0 to 3, the intra prediction mode of thechroma block may be derived as one of the planar mode, the verticalmode, the horizontal mode and the DC mode based on the value ofintra_chroma_pred_mode, and, when intra_chroma_pred_mode is 4, the intraprediction mode of the chroma block may be derived based on the value ofintra_chroma_pred_mode and the intra prediction mode of thecorresponding luma block. However, the present disclosure is not limitedto the above-described example, and may include an embodiment in which,even when intra_chroma_pred_mode is 0 to 3, the intra prediction mode ofthe chroma block is derived based on the intra chroma prediction modeinformation (e.g., intra_chroma_pred_mode) and the intra prediction modeof the corresponding luma block.

Table 4 shows a mapping table for intra prediction mode derivation ofthe chroma block when CCLM is available (sps_cclm_enabled_flag=1).

TABLE 4 IntraPredModeY[ xCb + cbWidth / 2 ][ yCb + cbHeight / 2 ]intra_chroma_pred_mode[ xCb ][ yCb ] 0 50 18 1 X ( 0 <= X <= 66 ) 0 66 00 0 0 1 50 66 50 50 50 2 18 18 66 18 18 3 1 1 1 66 1 4 81 81 81 81 81 582 82 82 82 82 6 83 83 83 83 83 7 0 50 18 1 X

Table 4 shows the case where CCLM is available and further includesmodes for signaling the CCLM in addition to the modes of Table 3. InTable 4, when intra_chroma_pred_mode is 4 to 6, these may indicateINTRA_LT_CCLM, INTRA_L_CCLM and INTRA_T_CCLM, respectively. In Table 4,the case where intra_chroma_pred_mode is 0 to 3 and 7 may correspond tothe case where intra_chroma_pred_mode is 0 to 4 in Table 3,respectively. That is, intra_chroma_pred_mode being 7 indicates that theintra prediction mode of the chroma block is derived as DM.

As described with reference to Tables 3 and 4, the intra prediction modeof the chroma block may be derived based on intra chroma prediction modeinformation (intra_chroma_pred_mode) and/or the intra prediction mode ofthe corresponding luma block. For example, when the intra chromaprediction mode information indicates the DM, the intra prediction modeof the chroma block may be determined to be the same as the intraprediction mode of the corresponding luma block.

FIG. 16 is a flowchart illustrating a conventional method of deriving anintra prediction mode of a chroma block based on a corresponding lumablock.

When an intra-predicted current chroma block is input (S1610), the intraluma prediction mode of the corresponding luma block may be obtained(S1620). The intra luma prediction mode may be, for example, obtainedfrom a predetermined position (center position) in the correspondingluma block, as described above. After the intra luma prediction mode isobtained, the intra prediction mode of the current chroma block may bederived by referring to Table 3 or 4 based on the intra chromaprediction mode information obtained from a bitstream and/or the intraluma prediction mode (S1630). Thereafter, the prediction block of thecurrent chroma block may be generated by performing intra predictionwith respect to the current chroma block using the derived intra chromaprediction mode (S1640).

In step S1630, when the intra chroma prediction mode informationindicates the DM, the intra prediction mode of the current chroma blockmay be derived as the same value as the intra luma prediction mode ofthe corresponding luma block. When the intra chroma prediction modeinformation indicates a mode other than the DM, the intra predictionmode of the current chroma block may be derived as one of the planarmode, the DC mode, the vertical mode, the horizontal mode and the CCLMbased on the intra chroma prediction mode information.

However, the conventional method described with reference to FIG. 16does not consider the case where the intra luma prediction mode of thecorresponding luma block is not present. For example, as describedabove, a luma block and a chroma block in one CTU may be split into thesame block tree structure (SINGLE_TREE) or an individual block treestructure (DUAL_TREE). In the case of the dual tree, the luma block andthe chroma block corresponding thereto may be encoded in the sameprediction mode or different prediction modes. That is, the luma blockcorresponding to the intra-predicted chroma block may be intra-predictedor may be encoded in another prediction mode. If the corresponding lumablock is not intra-predicted, in step S1620, the intra luma predictionmode of the corresponding luma block may not be obtained, and thus, forexample, there may be a problem in that the intra prediction mode of thechroma block intra-predicted in the DM may not be derived.

Hereinafter, embodiments of the present disclosure for solving theproblems of the conventional method will be described.

FIG. 17 is a flowchart illustrating an embodiment of the presentdisclosure of deriving an intra prediction mode of a chroma block basedon a corresponding luma block.

According to an embodiment of the present disclosure shown in FIG. 17,when the intra prediction mode of the corresponding luma block is notavailable, it is possible to solve the problem of the conventionalmethod by using a default intra prediction mode.

Referring to FIG. 17, first, an intra-predicted current chroma block isinput (S1710). Thereafter, it may be determined whether the intra lumaprediction mode of the corresponding luma block necessary to derive theintra prediction mode of the current chroma block is available (S1720).As described above, the intra luma prediction mode of the correspondingluma block may be obtained from a predetermined position (e.g., centerposition) of the corresponding luma block. Accordingly, determination ofstep S1720 may be performed by checking the prediction mode at thepredetermined position of the corresponding luma block. For example,when the prediction mode at the predetermined position of thecorresponding luma block is an intra mode, it may be determined that theintra luma prediction mode is available. Alternatively, when theprediction mode at the predetermined position of the corresponding lumablock is a mode other than the intra mode (e.g., IBC mode), it may bedetermined that the intra luma prediction mode is not available. Asanother example, determination of step S1720 may be made based onwhether the predetermined position of the corresponding luma block hasan intra luma prediction mode.

Upon determining that the intra luma prediction mode is available instep S1720, steps S1730, S1740 and S1760 may be sequentially performed.Steps S1730, S1740 and S1760 respectively correspond to steps S1620,S1630 and S1640 of FIG. 16 and thus a detailed description thereof willbe omitted.

Upon determining that the intra luma prediction mode is not available instep S1720, a default intra prediction mode may be used. That is, theintra prediction mode of the current chroma block may be derived basedon the intra chroma prediction mode information and/or the default intraprediction mode (S1750). In this case, instead of an unavailable intraluma prediction mode, a default intra prediction mode may be used. Forexample, when the intra chroma prediction mode information indicates aDM, the intra prediction mode of the current chroma block may be derivedas a default intra prediction mode. In addition, when the intra chromaprediction mode information indicates a mode other than the DM mode, theintra prediction mode of the current chroma block may be derived as oneof the planar mode, the DC mode, the vertical mode, the horizontal modeand the CCLM based on the intra chroma prediction mode information.Thereafter, the prediction block of the current chroma block may begenerated by performing intra prediction with respect to the currentchroma block using the derived intra chroma prediction mode (S1760).

In step S1750, as the default intra prediction mode, the DC mode or theplanar mode may be used. However, the present disclosure is not limitedthereto and a predefined intra prediction mode or an intra predictionmode signaled through a bitstream may be used as the default intraprediction mode.

FIG. 18 is a flowchart illustrating another embodiment of the presentdisclosure of deriving an intra prediction mode of a chroma block basedon a corresponding luma block.

According to the embodiment shown in FIG. 18, when the intra predictionmode of the corresponding luma block is not available, it is possible tosolve the above-described problem of the conventional method by using adefault intra prediction mode. In addition, according to the embodimentshown in FIG. 18, whether the intra prediction mode of the correspondingluma block is available is determined only when the tree structure ofthe current block is a dual tree.

Referring to FIG. 18, first, an intra-predicted current chroma block isinput (S1810). Thereafter, it is determined whether the tree structureof a current block is a dual tree structure (S1815). When the treestructure of the current block is not a dual tree structure, that is,when it is a single tree structure, since a chroma block and acorresponding luma block may be regarded as being equallyintra-predicted, it may be determined that the intra luma predictionmode of the corresponding luma block is available. Accordingly, in thiscase, determination of step S1820 may be skipped. In other words, whenthe tree structure of the current block is a single tree structure,steps S1830, S1840 and S1860 may be sequentially performed to generatethe prediction block of the current chroma block.

When the tree structure of the current block is a dual tree structure instep S1815, determination of step S1820 may be performed. Processingaccording to the result of determination of step S1820 is equal to thatdescribed with reference to FIG. 17 and thus a repeated descriptionthereof will be omitted. That is, steps S1820 to S1860 may correspond tosteps S1720 to S1760 of FIG. 17, respectively.

Determination of steps S1815 and S1820 of FIG. 18 may be a specificembodiment of step S1720 of FIG. 17.

FIG. 19 is a flowchart illustrating another embodiment of the presentdisclosure of deriving an intra prediction mode of a chroma block basedon a corresponding luma block.

According to the embodiment shown in FIG. 19, when the intra predictionmode of the corresponding luma block is not available, it is possible tosolve the problem of the conventional method by using a default intraprediction mode. In addition, according to the embodiment shown in FIG.19, whether the intra prediction mode of the corresponding luma block isavailable is determined only when the tree structure of the currentblock is a dual tree and IBC is allowed.

Referring to FIG. 19, first, an intra-predicted current chroma block isinput (S1910). Thereafter, it is determined whether the tree structureof the current block is a dual tree structure (S1915). When the treestructure of the current block is not a dual tree structure, that is,when it is a single tree structure, as described with reference to FIG.18, since the intra luma prediction mode of the corresponding luma blockis regarded as being available, determination of steps S1917 and S1920may be skipped. Accordingly, in the case of the single tree structure,steps S1930, S1940 and S1960 may be sequentially performed to generatethe prediction block of the current chroma block.

When the tree structure of the current block is a dual tree structure instep S1915, it may be determined whether IBC is allowed (S1917). WhetherIBC is allowed may be determined based on information (e.g.,sps_ibc_enabled_flag) signaled at a higher level (e.g., sequence) of thecurrent block. For example, when sps_ibc_enabled_flag is 1, it may bedetermined that IBC is allowed and, when sps_ibc_enabled_flag is 0, itmay be determined that IBC is not allowed.

As described above, only a single tree may be allowed for P and Bslice/tile groups, and both a single tree and a dual tree may be allowedfor I slice/tile groups. Accordingly, when the tree structure of thecurrent block is a dual tree structure, the current block belongs to anI slice/tile group. When it is assumed that prediction for a blockincluded in the I slice/tile group is limited to intra prediction andIBC prediction, the current block may be predicted as one of intraprediction or IBC prediction. Accordingly, when IBC is not allowed, itmay be determined that the current block is intra-predicted. That is,upon determining that IBC is not allowed in step S1917, since thecorresponding luma block may be regarded as being intra-predicted,determination of step S1920 may be skipped and steps S1930, S1940 andS1960 may be immediately performed. Upon determining that IBC is allowedin step S1917, since the corresponding luma block may be intra-predictedor IBC-predicted, in this case, determination of step S1920 may beperformed. Processing according to the result of determination of stepS1920 is equal to that described with reference to FIG. 17 and thus arepeated description thereof will be omitted. That is, steps S1920 toS1960 may correspond to steps S1720 to S1760 of FIG. 17, respectively.

In the example shown in FIG. 19, since it is assumed that only intraprediction or IBC prediction is possible for blocks belonging to the Islice/tile group, it is only determined that IBC is allowed in stepS1917. However, when a third prediction mode is possible for blocksbelonging to the I slice/tile group in addition to intra prediction orIBC prediction, not only whether IBC is allowed but also whether thethird prediction mode is allowed may be determined in step S1917. Inthis case, step S1920 may be skipped only when both IBC and the thirdprediction mode are not allowed. That is, when any one of IBC and thethird prediction mode is allowed, step S1920 and a subsequent processthereof may be performed as described above.

Determination of steps S1915, S1917 and S1920 of FIG. 19 may be aspecific embodiment of step S1720 of FIG. 17.

As modified examples of FIGS. 18 and 19, prior to determining whetherthe tree structure of the current block is a dual tree structure, thetype of the slice/tile group of the current block may be determined.When the type of the slice/tile group of the current block is a P or Bslice/tile group, since the tree structure of the current block isdetermined to be a single tree structure, it is not necessary todetermine whether the tree structure of the current block is a dual treestructure (S1815 or S1915). Accordingly, in the case of the P or Bslice/tile group, the corresponding intra luma prediction mode may beimmediately obtained and then the intra chroma prediction mode may bederived based on this. In addition, when the type of the slice/tilegroup of the current block is an I slice/tile group, since the treestructure of the current block may be a single tree structure or a dualtree structure, steps of FIGS. 18 and 19 including S1815 or S1915 may beperformed.

FIG. 20 is a flowchart illustrating another embodiment of the presentdisclosure of deriving an intra prediction mode of a current block basedon a corresponding block.

First, a current block to be encoded/decoded may be input (S2010). Thecurrent block is an intra-predicted block and the intra prediction modeof the current block may be derived from the intra prediction mode of acorresponding block without being directly signaled. For example, thecurrent block may be a chroma block. When the current block is input, acorresponding block corresponding to the current block may be specifiedand whether the intra prediction mode of the corresponding block isavailable may be determined (S2020). For example, the correspondingblock may be a corresponding luma block corresponding to a chroma block.Step S2020 may be performed based on determination of whether thecorresponding block is intra-predicted. Alternatively, step S2020 may beperformed based on the determination of whether the prediction mode ofthe corresponding block is a mode (e.g., IBC mode) other than the intramode. Alternatively, step S2020 may be performed based on determinationof whether the corresponding block has an intra prediction mode. Forexample, when the corresponding block is intra-predicted or has an intraprediction mode, it may be determined that the intra prediction mode ofthe corresponding block is available. When the corresponding block ispredicted in a mode other than the intra mode or does not have an intraprediction mode, it may be determined that the intra prediction mode ofthe corresponding block is not available. Alternatively, as describedabove, step S2020 may include at least one of determination of the typeof the slice/tile group including the current block, determination ofthe tree structure of the current block, or determination of whether theIBC or third prediction mode is allowed for the current block inaddition to the intra prediction.

Although the current block and the corresponding block are exemplifiedas the chroma block and the corresponding luma block, the presentdisclosure is not limited thereto. For example, the current block may bea luma block and the corresponding block may be a corresponding chromablock. Alternatively, the current block is a first color component blockand the corresponding bock may be a second color component block. Inthis case, the first color component may be, for example, one of a lumacomponent and a plurality of chroma components, and the second colorcomponent may be a color component different from the first colorcomponent. For example, the first color component block may be a firstchroma component block, and the second color component block may be asecond chroma component block.

Referring to FIG. 20 again, when the intra prediction mode of thecorresponding block is available in step S2020, the intra predictionmode of the corresponding block may be obtained (S2030), and the intraprediction mode of the current block may be derived based on the intraprediction mode of the corresponding block (S2040). In step S2020, whenthe intra prediction mode of the corresponding block is not available, adefault intra prediction mode may be determined (S2050), and the intraprediction mode of the current block may be derived based on the defaultintra prediction mode (S2060). As the default intra prediction mode ofstep S2050, a DC mode or a planar mode may be used. However, the presentdisclosure is not limited thereto and, as the default intra predictionmode, a predefined intra prediction mode or an intra prediction modesignaled through a bitstream may be used.

According to the present disclosure, when the intra prediction mode ofthe current block is derived from the intra prediction mode of thecorresponding block, if the intra prediction mode of the correspondingblock is not present, since the intra prediction mode of the currentblock may be derived based on the default intra prediction mode, it ispossible to efficiently perform intra prediction with respect to thecurrent block even if the intra prediction mode of the correspondingblock is not present.

FIG. 21 is a flowchart illustrating another embodiment of the presentdisclosure of deriving an intra prediction mode of a chroma block basedon a corresponding luma block and encoding the chroma block.

FIG. 21 relates to operation of an image encoding apparatuscorresponding to the embodiment described with reference to FIG. 17.

Referring to FIG. 21, first, a current chroma block is input (S2110). Inorder to perform intra prediction with respect to the current chromablock, the image encoding apparatus may determine the intra predictionmode of the current chroma block. The image encoding apparatus mayperform intra prediction with respect to all or at least some of theintra prediction modes applicable to the current chroma block anddetermine an optimal mode as the intra prediction mode of the currentchroma block. Accordingly, the image encoding apparatus needs todetermine whether to apply a DM to the current chroma block. To thisend, it is necessary to obtain the intra prediction mode of acorresponding luma block. However, since the intra prediction mode ofthe corresponding luma block may not be available, the image encodingapparatus may determine whether the intra luma prediction mode of thecorresponding luma block is available (S2120). Determination of stepS2120 is substantially equal to determination of step S1720 and thus arepeated description will be omitted. Alternatively, as described above,step S2120 may include at least one of determination of the type of aslice/tile group including the current block, determination of the treestructure of the current block or determination of whether IBC or athird prediction mode is allowed for the current block in addition tointra prediction.

In step S2120, upon determining that the intra luma prediction mode isavailable, the image encoding apparatus may obtain the intra lumaprediction mode from a predetermined position of a corresponding lumablock (S2130), and derive the intra prediction mode of the currentchroma block including the intra luma prediction mode (S2140). Forexample, the image encoding apparatus may select one intra predictionmode from among one or more intra prediction modes applicable to thecurrent chroma block including the intra luma prediction mode bycomparison of the RD-cost. However, a method of selecting the intraprediction mode of the current chroma block by the image encodingapparatus is not limited to the above-described example. When the intraprediction mode of the current chroma block is derived, intra predictionmay be performed with respect to the current chroma block based on thederived intra prediction mode, and a prediction block may be generated.In addition, the intra prediction mode of the current chroma block maybe encoded based on the intra luma prediction mode (S2160). For example,when the intra luma prediction mode is determined to be the intraprediction mode of the current chroma block, the intra chroma predictionmode information may be encoded into a value indicating the DM.

When the intra luma prediction mode of the corresponding luma block isnot available in step S2120, instead of the intra luma prediction mode,the intra prediction mode of the current chroma block may be derivedusing a default intra prediction mode (S2150). When the intra predictionmode of the current chroma block is derived, intra prediction may beperformed with respect to the current chroma block based on the derivedintra prediction mode, and a prediction block may be generated. Inaddition, the intra prediction mode of the current chroma block may beencoded based on the default intra prediction mode (S2160).

In step S2150, a DC mode or a planar mode may be used as the defaultintra prediction mode. However, the present disclosure is not limitedthereto, a predefined intra prediction mode may be used as a defaultintra prediction mode, or the default intra prediction mode may bedetermined by the image encoding apparatus and information on thedetermined default intra prediction mode may be signaled through abitstream.

While the exemplary methods of the present disclosure described aboveare represented as a series of operations for clarity of description, itis not intended to limit the order in which the steps are performed, andthe steps may be performed simultaneously or in different order asnecessary. In order to implement the method according to the presentdisclosure, the described steps may further include other steps, mayinclude remaining steps except for some of the steps, or may includeother additional steps except for some steps.

In the present disclosure, the image encoding apparatus or the imagedecoding apparatus that performs a predetermined operation (step) mayperform an operation (step) of confirming an execution condition orsituation of the corresponding operation (step). For example, if it isdescribed that predetermined operation is performed when a predeterminedcondition is satisfied, the image encoding apparatus or the imagedecoding apparatus may perform the predetermined operation afterdetermining whether the predetermined condition is satisfied.

The various embodiments of the present disclosure are not a list of allpossible combinations and are intended to describe representativeaspects of the present disclosure, and the matters described in thevarious embodiments may be applied independently or in combination oftwo or more.

Various embodiments of the present disclosure may be implemented inhardware, firmware, software, or a combination thereof. In the case ofimplementing the present disclosure by hardware, the present disclosurecan be implemented with application specific integrated circuits(ASICs), Digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), general processors, controllers, microcontrollers,microprocessors, etc.

In addition, the image decoding apparatus and the image encodingapparatus, to which the embodiments of the present disclosure areapplied, may be included in a multimedia broadcasting transmission andreception device, a mobile communication terminal, a home cinema videodevice, a digital cinema video device, a surveillance camera, a videochat device, a real time communication device such as videocommunication, a mobile streaming device, a storage medium, a camcorder,a video on demand (VoD) service providing device, an OTT video (over thetop video) device, an Internet streaming service providing device, athree-dimensional (3D) video device, a video telephony video device, amedical video device, and the like, and may be used to process videosignals or data signals. For example, the OTT video devices may includea game console, a blu-ray player, an Internet access TV, a home theatersystem, a smartphone, a tablet PC, a digital video recorder (DVR), orthe like.

FIG. 22 is a view showing a content streaming system, to which anembodiment of the present disclosure is applicable.

As shown in FIG. 22, the content streaming system, to which theembodiment of the present disclosure is applied, may largely include anencoding server, a streaming server, a web server, a media storage, auser device, and a multimedia input device.

The encoding server compresses content input from multimedia inputdevices such as a smartphone, a camera, a camcorder, etc. into digitaldata to generate a bitstream and transmits the bitstream to thestreaming server. As another example, when the multimedia input devicessuch as smartphones, cameras, camcorders, etc. directly generate abitstream, the encoding server may be omitted.

The bitstream may be generated by an image encoding method or an imageencoding apparatus, to which the embodiment of the present disclosure isapplied, and the streaming server may temporarily store the bitstream inthe process of transmitting or receiving the bitstream.

The streaming server transmits the multimedia data to the user devicebased on a user's request through the web server, and the web serverserves as a medium for informing the user of a service. When the userrequests a desired service from the web server, the web server maydeliver it to a streaming server, and the streaming server may transmitmultimedia data to the user. In this case, the content streaming systemmay include a separate control server. In this case, the control serverserves to control a command/response between devices in the contentstreaming system.

The streaming server may receive content from a media storage and/or anencoding server. For example, when the content are received from theencoding server, the content may be received in real time. In this case,in order to provide a smooth streaming service, the streaming server maystore the bitstream for a predetermined time.

Examples of the user device may include a mobile phone, a smartphone, alaptop computer, a digital broadcasting terminal, a personal digitalassistant (PDA), a portable multimedia player (PMP), navigation, a slatePC, tablet PCs, ultrabooks, wearable devices (e.g., smartwatches, smartglasses, head mounted displays), digital TVs, desktops computer, digitalsignage, and the like.

Each server in the content streaming system may be operated as adistributed server, in which case data received from each server may bedistributed.

The scope of the disclosure includes software or machine-executablecommands (e.g., an operating system, an application, firmware, aprogram, etc.) for enabling operations according to the methods ofvarious embodiments to be executed on an apparatus or a computer, anon-transitory computer-readable medium having such software or commandsstored thereon and executable on the apparatus or the computer.

INDUSTRIAL APPLICABILITY

The embodiments of the present disclosure may be used to encode ordecode an image.

1. An image decoding method performed by an image decoding apparatus,the image decoding method comprising: determining whether to apply intraprediction to a current chroma block based on information on predictionof the current chroma block; deriving an intra prediction mode of thecurrent chroma block based on an intra prediction mode of acorresponding luma block corresponding to the current chroma block andintra chroma prediction mode information of the current chroma block,when intra prediction applies to the current chroma block; andgenerating a prediction block of the current chroma block, by performingintra prediction based on the intra prediction mode of the currentchroma block, wherein, when the intra prediction mode of thecorresponding luma block is not present, the intra prediction mode ofthe current chroma block is derived based on a default intra predictionmode.
 2. The image decoding method of claim 1, wherein the deriving theintra prediction mode of the current chroma block comprises determininga prediction method at a predetermined position of the correspondingluma block.
 3. The image decoding method of claim 2, wherein, when theprediction method at the predetermined position of the correspondingluma block is intra prediction, the intra prediction mode of the currentchroma block is derived based on an intra prediction mode at apredetermined position of the corresponding luma block, and wherein,when the prediction method at the predetermined position of thecorresponding luma block is not intra prediction, the intra predictionmode of the current chroma block is derived based on the default intraprediction mode.
 4. The image decoding method of claim 2, wherein, whenthe prediction method at the predetermined position of the correspondingluma block is intra block copy (IBC) prediction, the intra predictionmode of the current chroma block is derived based on the default intraprediction mode.
 5. The image decoding method of claim 2, wherein thepredetermined position is a center position of the corresponding lumablock.
 6. The image decoding method of claim 1, wherein the defaultintra prediction mode is a planar mode or a DC mode.
 7. The imagedecoding method of claim 1, wherein a tree structure of the currentchroma block is a dual tree (DUAL_TREE) structure.
 8. The image decodingmethod of claim 1, wherein, when the intra prediction mode informationof the current chroma block indicates a direct mode (DM) and the intraprediction mode of the corresponding luma block is present, the intraprediction mode of the current chroma block is derived as the intraprediction mode of the corresponding luma block, and wherein, when theintra prediction mode information of the current chroma block indicatesa DM and the intra prediction mode of the corresponding luma block isnot present, the intra prediction mode of the current chroma block isderived as the default intra prediction mode.
 9. An image decodingapparatus comprising: a memory; and at least one processor, wherein theat least one processor is configured to: determine whether to applyintra prediction to a current chroma block based on information onprediction of the current chroma block, derive an intra prediction modeof the current chroma block based on an intra prediction mode of acorresponding luma block corresponding to the current chroma block andintra chroma prediction mode information of the current chroma block,when intra prediction applies to the current chroma block, and generatea prediction block of the current chroma block, by performing intraprediction based on the intra prediction mode of the current chromablock, wherein, when the intra prediction mode of the corresponding lumablock is not present, the intra prediction mode of the current chromablock is derived based on a default intra prediction mode.
 10. An imageencoding method performed by an image encoding apparatus, the imageencoding method comprising: determining whether to apply intraprediction to a current chroma block; deriving an intra prediction modeof the current chroma block based on an intra prediction mode of acorresponding luma block corresponding to the current chroma block, whenintra prediction applies to the current chroma block; generating aprediction block of the current chroma block, by performing intraprediction based on the intra prediction mode of the current chromablock; and encoding the intra prediction mode of the current chromablock based on the intra prediction mode of the corresponding lumablock, wherein, when the intra prediction mode of the corresponding lumablock is not present, the intra prediction mode of the current chromablock is derived based on a default intra prediction mode.
 11. The imageencoding method of claim 10, wherein the deriving the intra predictionmode of the current chroma block comprises determining a predictionmethod at a predetermined position of the corresponding luma block. 12.The image encoding method of claim 11, wherein, when the predictionmethod at the predetermined position of the corresponding luma block isintra prediction, the intra prediction mode of the current chroma blockis derived based on an intra prediction mode at a predetermined positionof the corresponding luma block, and wherein, when the prediction methodat the predetermined position of the corresponding luma block is notintra prediction, the intra prediction mode of the current chroma blockis derived based on the default intra prediction mode.
 13. The imageencoding method of claim 11, wherein the predetermined position is acenter position of the corresponding luma block.
 14. The image encodingmethod of claim 10, wherein the default intra prediction mode is aplanar mode or a DC mode.
 15. A method of transmitting a bitstreamgenerated by the image encoding method of claim 10.